Hypoxia-Inducible transcription factors (HIFs) are thought to regulate multiple facets of solid tumor progression. Using genetic and biochemical screens for proteins able to modify HIF-1alpha responses, we have isolated the CSN5 subunit of the COP9 Signalosome (CSN). We have further found that CSN5 associated directly and constitutively with the pVHL tumor suppressor. This proposal seeks to define the precise mechanism(s) in which CSN5 regulates pVHL-mediated HIF-alpha stability. To accomplish this we propose to: 1) test if CSN5 regulates global or specific 26S-dependent protein degradation; 2) test if CSN5 binds HIF-1a or pVHL in association with the CSN holocomplex or as a component of another protein complex and which CSN5-associated complex (es) are able to prevent HIF-la ubiquitylation; 3) test if mutations in CSN components that influence CSN assembly or activity can alter HIF-la stability; and 4) examine the mechanism(s) by which CSN5 directly alters HIF-1a stabilization. Accomplishment of these specific aims will reveal novel pathways of HIF-1alpha and pVHL regulation and will suggest potential mechanisms by which these pathways could be manipulated for therapeutic benefit.